function [vx, vf] = fn_prod(vx1, vf1, vx2, vf2)

vx = [vx1, vx2];
vx = sort(vx);
vx = unique(vx);

vf1_ins = interp1(vx1, vf1, vx, 'pchip');
vf2_ins = interp1(vx2, vf2, vx, 'pchip');
vf = vf1_ins.*vf2_ins;

end